[FEATURE REQ] Add description of how to override the default TokenCredential bean

[saragluna]

Is your feature request related to a problem? Please describe.
In Spring Cloud Azure 4.0 we provided a default TokenCredential bean named AzureContextUtils.DEFAULT_TOKEN_CREDENTIAL_BEAN_NAME and users could provide their own bean with this name to override the default one. We should mention it in our doc.

Describe the solution you'd like
Add a section in our refdoc on how to use it.

Describe alternatives you've considered
N/A

Additional context
N/A

Information Checklist

• Description Added
• Expected solution specified

[Netyyyy]

close for moving to current repo