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

[Netyyyy]

Is your feature request related to a problem? Please describe.
Transferred from microsoft/spring-cloud-azure#989

In Spring Cloud Azure 4.x/5.x 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

[github-actions]

@moarychan @Netyyyy @rujche @saragluna

[github-actions]

Thank you for your feedback. Tagging and routing to the team member best able to assist.