xds: httpfilter API changes to support filter state retention (A83)

[easwars]

How the API looked before this change?

• We had a single interface, Filter, that was responsible for functionality like config parsing and optionally for building client and server interceptors (or filter instances).
• We had a registry of Filters keyed by the supported type_urls.

How the API looks after this change?

• We will have an interface, FilterProvider, that contains functionality for config parsing and other things like "what are the supported type_urls?", "Are filters produced by this provider supposed to be terminal?", "Are filters produced by this provider supposed to work on the client/server?" etc.
• The FilterProvider will also contain functionality to create filter instances represented by a new interface, Filter.
• The Filter interface contains functionality to build client and server interceptors. If this filter is not supported on either the client or the server, that functionality can be a no-op.
• The Filter will also contain a new Close method allowing it to free up any resources allocated.
• We will have a registry of FilterProviderss keyed by the supported type_urls.

Why is this change required?

• As part of gRFC A83 and moving forward on a bunch of other gRFCs, we will start having HTTP filters that need to maintain a bunch of state, and this state needs to be retained across resource updates. For example, the filter might contain a gRPC channel to an external service, and we don't want to recreate this channel for resource updates that doesn't change any properties associated with the service that is being connected to.
  • This change is not part of this PR and will be made when the first filter that needs state retention support is implemented.

How will this be used?

• The xDS name resolver and the xDS enabled grpc server (entities that create HTTP filters on the client and server side) will create new Filter instances only when the filter names in the xDS resources change. Otherwise, they will only create new interceptors using the updated filter configuration from the existing Filter. This will allow Filter instances to share state across interceptors and across state updates.

The changes here and inspired by similar changes made for Java and described here: https://github.com/grpc/proposal/blob/master/A83-xds-gcp-authn-filter.md#java

RELEASE NOTES: None

[codecov]

Codecov Report

❌ Patch coverage is 43.47826% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.90%. Comparing base (647162c) to head (7ebfeaa).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/httpfilter/router/router.go	25.00%	6 Missing and 6 partials ⚠️
internal/xds/httpfilter/fault/fault.go	26.66%	7 Missing and 4 partials ⚠️
internal/xds/httpfilter/rbac/rbac.go	26.66%	6 Missing and 5 partials ⚠️
internal/xds/resolver/serviceconfig.go	33.33%	0 Missing and 2 partials ⚠️
internal/xds/xdsclient/xdsresource/filter_chain.go	33.33%	0 Missing and 2 partials ⚠️
...nternal/xds/xdsclient/xdsresource/unmarshal_lds.go	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8745      +/-   ##
==========================================
- Coverage   83.38%   82.90%   -0.49%     
==========================================
  Files         418      418              
  Lines       32367    32392      +25     
==========================================
- Hits        26988    26853     -135     
- Misses       4014     4038      +24     
- Partials     1365     1501     +136     

Files with missing lines	Coverage Δ
internal/xds/httpfilter/httpfilter.go	87.50% <100.00%> (-12.50%)	⬇️
...nternal/xds/xdsclient/xdsresource/unmarshal_lds.go	87.82% <93.33%> (-0.65%)	⬇️
internal/xds/resolver/serviceconfig.go	87.21% <33.33%> (-0.76%)	⬇️
internal/xds/xdsclient/xdsresource/filter_chain.go	71.27% <33.33%> (-23.21%)	⬇️
internal/xds/httpfilter/fault/fault.go	41.09% <26.66%> (-30.88%)	⬇️
internal/xds/httpfilter/rbac/rbac.go	50.63% <26.66%> (-9.94%)	⬇️
internal/xds/httpfilter/router/router.go	25.00% <25.00%> (-13.24%)	⬇️

... and 15 files with indirect coverage changes

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