Managed Identity Support for Pg

[gledis69]

Why make this change?

• Add extra support for Azure Managed Identity for Pg. I say extra, because even in the repo's current state, the user can just specify the host and pass the access token as the password in the connection string to authenticate to their managed PG with a managed identity.
• The extra part is:
  • Now the user can alternatively specify the access token in the config to authenticate with managed identity.
  • Alternatively, now the user can just not specify the password in the connection string and the runtime will attempt to fetch the default managed identity token. If this fails, connection will be attempted without a password in the connection string.

What is this change?

• Introduces a class PostgreSqlQueryExecutor inspired by how managed identity was supported for Sql Server.

How was this tested?

• Unit Tests

[seantleonard]

one nit, otherwise looks good :)

[seantleonard]

I think it's fair to denote USE DEFAULT for deferring to system assigned managed identity. The behavior is needed, though if we want to enhance in the future, we could consider updating the runtime configuration schema to ask a user to use a different JSON value like true

[Aniruddh25]

Looks good for hosted scenario, but unclear waiting on how "USE DEFAULT" will be set by user and identified by RuntimeConfigProvider

[Aniruddh25]

Through offline discussion, it was decided to tackle this as follows:
If password is absent in the connection string, can we imply in decreasing order of preference:

1. Managed Identity scenario - attempt to get the token
2. Passwordless Pg User
   eventually, fail startup of DAB if none of the scenarios succeed auth

I will again be OOF starting 12/12, dont want my review to block the merge of this PR

[Aniruddh25]

Thank you for pushing through this change for the Nov2022 release, really appreciate it!
Left 1 optimization for passwordless scenarios, otherwise LGTM.