Feat/workload identity support

[jonasz-lasut]

Description of your changes

PR implements support for short-lived workload identity credentials that can be used in AKS clusters with workload identity enabled and properly created federated identity credentials. (https://azure.github.io/azure-workload-identity/docs/topics/federated-identity-credential.html)
Default behavior of the function which expects ServicePrincipal credentials have not been changed.
README was updated to show how to configure DeploymentRuntimeConfig so that function can use workload identity credentials.
Function was tested on Upbound Spaces 1.13.1 with Crossplane in version 1.20.0-up.1

I have:

• Read and followed Crossplane's contribution process.
• Added or updated unit tests for my change.

[ytsarev]

I built and pushed rc build of xpkg.upbound.io/upbound/function-msgraph:v0.3.0-rc3 and performed e2e tests.

k apply -f example/secrets/azure-workload-identity-creds.yaml
k apply -f example/identity-type-workload-identity.yaml
k apply -f example/definition.yaml # comment status fields there
k get -f example/xr.yaml -o yaml
...
  validatedUsers:
  - displayName: Yury Tsarev
    id: <redacted>
    mail: null
    userPrincipalName: yury@upbound.io

everything works perfectly end-to-end.

@jonasz-lasut thank you so much for this contribution!