feat: bedrock token provider

[stobrien89]

Description of changes:
Adds a provider to fetch token values from environment. If found, sets auth scheme preference to prefer bearer. Additional tests will be added once #3156 is approved and merged.

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[yenfryherrerafeliz]

Looks good. Just some minor changes and some questions.

Thanks!

[yenfryherrerafeliz]

Clean implementation. Just a question and two minor asks.

Thanks!

[stobrien89]

Smoke test run:

vendor/bin/behat --format=progress --suite=smoke --tags='~@noassumerole'
...................................................................... 70
...................................................................... 140
...................................................................... 210
............

111 scenarios (111 passed)
222 steps (222 passed)
0m30.58s (95.24Mb)

Integ test run:

vendor/bin/behat --format=progress --tags=integ
...................................................................... 70
...................................................................... 140
...................................................................... 210
......................................

66 scenarios (66 passed)
248 steps (248 passed)
8m4.51s (48.33Mb)