Bridged tokens from AMB extensions support

[vbaranov]

Motivation

Add bridged tokens from AMB extensions to the bridged tokens page and fetch their metadata.

Changelog

AMB_BRIDGE_MEDIATORS environment variable is added which is a comma-separated list of AMB extensions mediators

Chore:

MULTI_TOKEN_BRIDGE_MEDIATOR env var renamed to OMNI_BRIDGE_MEDIATOR

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.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 352ee9d7bf781b9d73bd0bdfc31589189af41188-PR-3320

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 16 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+2.7%) to 74.398%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/channels/address_channel.ex	2	36.96%
lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%
lib/block_scout_web/controllers/tokens/bridged_tokens_controller.ex	3	0%
lib/block_scout_web/views/bridged_tokens_view.ex	3	0%
lib/block_scout_web/checksum_address.ex	6	55.56%

Totals
Change from base Build 13f76c1c41a70edf901b444fc90c27543841725f:	2.7%
Covered Lines:	1976
Relevant Lines:	2656

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💛 - Coveralls