Add possibility to add extra headers with JSON RPC URL

[sl1depengwyn]

Close #7645

Changelog

Add ETHEREUM_JSONRPC_HTTP_HEADERS env variable that accepts json object with headers
Example:

{
  "x-api-key": "nbvkhadvnbkdfav",
  "x-id": "ndjkfvndfkjv"
}

will result as following headers in requests:

  Content-Type: application/json
  x-api-key: nbvkhadvnbkdfav
  x-id: ndjkfvndfkjv

Content-Type header is added automatically, however if it appears in env variable, its value will be taken from env variable

If ETHEREUM_JSONRPC_HTTP_HEADERS is not valid json app wouldn't start

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables. (Add ETHEREUM_JSONRPC_HTTP_HEADERS env docs#167)
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools