Enable OAuth2 authentication

[ojecborec]

Hello OPAL team. Following PR is enabling OAuth2 authentication for both server and client. Client would send access token generated by Client Credentials grant and server would validate this token by either calling introspect endpoint or reading JWT signature.
Please note that I'm not python developer. Any suggestions are welcomed. There's also missing documentation.

To configure OPAL client Docker container.

environment:
  - OPAL_AUTH_TYPE=oauth2
  - OPAL_OAUTH2_CLIENT_ID=$OAUTH2_CLIENT_ID
  - OPAL_OAUTH2_CLIENT_SECRET=$OAUTH2_CLIENT_SECRET
  - OPAL_OAUTH2_TOKEN_URL=https://example.com/token
  # Choose either introspect or JWT validation
  # If you wish to use introspect endpoint for token validation
  - OPAL_OAUTH2_INTROSPECT_URL=https://example.com/introspect
  # Validate JWT signature instead of calling introspect endpoint
  - OPAL_OAUTH2_OPENID_CONFIGURATION_URL=https://example.com/.well-known/openid-configuration
  - OPAL_OAUTH2_EXACT_MATCH_CLAIMS=foo=bar
  - OPAL_OAUTH2_REQUIRED_CLAIMS=client_id,iat,exp

To configure OPAL server Docker container.

environment:
  - OPAL_AUTH_TYPE=oauth2
  - OPAL_OAUTH2_TOKEN_URL=https://example.com/token
  # Choose either introspect or JWT validation
  # If you wish to use introspect endpoint for token validation
  - OPAL_OAUTH2_INTROSPECT_URL=https://example.com/introspect
  # Validate JWT signature instead of calling introspect endpoint
  - OPAL_OAUTH2_OPENID_CONFIGURATION_URL=https://example.com/.well-known/openid-configuration
  - OPAL_OAUTH2_EXACT_MATCH_CLAIMS=foo=bar
  - OPAL_OAUTH2_REQUIRED_CLAIMS=client_id,iat,exp
  - OPAL_OAUTH2_JWT_ALGORITHM=RS256
  - OPAL_OAUTH2_JWT_AUDIENCE=some_audience
  - OPAL_OAUTH2_JWT_ISSUER=some_issuer

Let me know what you think.

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[roekatz]

@ojecborec Rebasing on top of master as @asafc suggested, should also fix the missing docker-compose issue (as it was already fixed in master). And of course enable us to review the change (cause currently most of the diff is master changes rather than your own branch changes).

[ojecborec]

Hello @roekatz @asafc. It was easier for me to create the new branch. See #646 for reference.

[roekatz]

Replaced by #646