Add support for configuring username and password fields for select DB Engines

[vinay-gopalan]

This PR adds support for configuring the username and password fields in Vault through the Provider for all DB Engines that support those fields (all DBs except MongoDB Atlas)

Relates OR Closes #1314 #1329

Release note for CHANGELOG:

`resource/database_secret_backend_connection`: Add `username` and `password` fields to all DB engines that support them

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccDatabaseSecretBackendConnection_mssql'
=== RUN   TestAccDatabaseSecretBackendConnection_mssql
--- PASS: TestAccDatabaseSecretBackendConnection_mssql (12.10s)
PASS

$ make testacc TESTARGS='-run=TestAccDatabaseSecretBackendConnection_postgresql'
=== RUN   TestAccDatabaseSecretBackendConnection_postgresql
--- PASS: TestAccDatabaseSecretBackendConnection_postgresql (12.10s)

make testacc TESTARGS='-run=TestAccDatabaseSecretBackendConnection_mysql'
=== RUN   TestAccDatabaseSecretBackendConnection_mysql
--- PASS: TestAccDatabaseSecretBackendConnection_mysql (5.52s)
PASS

[benashz]

Looks Great!

[benashz]

So these are unrelated to this PR?

[vinay-gopalan]

These are to be removed since username and password are now being set on the resource config itself, though how these data fields are passed to Vault, I am slightly unclear on

[benashz]

Ship it!