Add custom text document content provider

[evidolob]

What does this PR do?

Add implementation for text document content provider which handle json-schema URI shema, to open JSON Schema in VScode editor

Note: to be able to open files from LS this PR also update vscode-languageclient to 7.0.0 version

What issues does this PR fix or reference?

#174

Is it tested? How?

[JPinkney]

I like this approach because it means that we won't need to manually download schemas ourselves to show remote schemas. I feel like handling remote schemas, downloading them, making sure they are updated, etc would be a pain.

I guess in the end the only difference is that users on different ides/editors can't jump to remote schemas but I'm not sure how big of a deal that is

[evidolob]

@JPinkney I was thinking for adding some sort cache for remote JSON Schemas, we already have issues to support offline work - #386
Now as we load schema from vscode extension, we can cache them in to ExtensionContex#storageUri.
To keep cache valid, we can check file creation/modification time and if it was more then (for example) 1 week ago we download that file again. In ideal case we can add that time in to preference.
@JPinkney WDYT?

[JPinkney]

That sounds like a good approach!

[evidolob]

@JPinkney Can you look on this PR again?

I also find that we use yaml/registerVSCodeContentRequest type, but yaml-LS expects yaml/registerContentRequest, so I change that to be alight with yaml-ls

[gorkem]

would using eTag make the cache more efficient?

[evidolob]

@gorkem It can, I'm was thinking on rewriting caching on this PR.

[evidolob]

@gorkem @JPinkney
Now cache uses etag header to detect that schema changed on server.

[JPinkney]

LGTM!