Add a Helm chart parameter for the SSH secret's data key

[sbernheim]

Describe the feature
Add a git.secretDataKey parameter to the Helm chart that enables the user to override the default deploy key secret's data key identity. They can then create the secret from a file with a different name (like id_rsa) or to use a pre-existing secret that stores the deploy key's secret key value under a different data key.

What would the new user story look like?
How would the new interaction with Flux look like? E.g.

1. User either creates a secret to store their deploy key's private key using an existing id_rsa key file or decides to use a deploy key stored in an existing secret.
2. User adds a secretDataKey field to the Helm chart's values.yaml to override the default key name identity.
3. User deploys Flux using the Helm chart and their values.yaml overrides.
4. Flux connects to Git using the private key stored in the Secret described by git.secretName and the data key described by git.secretDataKey values in the values.yaml.

Expected behavior
The Flux Helm chart should enable the user to set the data key of the deploy key secret in a git.secretDataKey field in their values.yaml.