Support Event Indexing

[alexanderbez]

Description

Add a new index_events config/flag to support application-driven event filtering as this will be removed in Tendermint 0.34.

/cc @marbar3778

closes: #6277

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #7121 into master will increase coverage by 0.00%.
The diff coverage is 67.85%.

@@           Coverage Diff           @@
##           master    #7121   +/-   ##
=======================================
  Coverage   54.21%   54.22%           
=======================================
  Files         547      547           
  Lines       37538    37564   +26     
=======================================
+ Hits        20351    20368   +17     
- Misses      15508    15517    +9     
  Partials     1679     1679           

[fedekunze]

LGTM. Pending changelog entry