Allow passing basic auth credentials source.

[chasdevs]

We are using aiven.io for hosting schema registry, which uses basic auth. This PR adds a parameter for basic.auth.credentials.source and passes it to the CachedSchemaRegistry.

Ex:

    val schemaRegistryConf = Map(
      SchemaManager.PARAM_SCHEMA_REGISTRY_URL -> "https://user:password@schema.url",
      SchemaManager.PARAM_SCHEMA_REGISTRY_BASIC_AUTH_SOURCE -> "URL"
    )

...

[bitsofdave]

@chasdevs Were you able to use basic authentication without this PR? If so, could you please provide any pointers on how?