AccessPolicyToken: Add basic auth credentials to ConnectionSecret for use with DataSource

[holgerjh]

Description of your changes

This PR changes the connection secret created by accesspolicytokens.cloud.grafana.crossplane.io so that it contains an additional key that can be used by datasources (datasources.oss.grafana.crossplane.io) that make use of basic authentication.

In detail, a new key basicAuthCredentials is added to the connection secret. It contains a stringified JSON-object with the following content:

{"basicAuthPassword": "<TOKEN>"}

We also provided an example to see how it can be used.

Fixes #238

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

• Run the provider locally and applied the example (with replaced "changeme" fields).
• Checked that the datasource was properly created in grafana cloud (basic auth password configured and connection working)

[codecov-commenter]

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