Add new configurable bootstrap writers (#2405) (#5114)

* Add new configurable bootstrap writers (#2405)

This PR adds the ability to configure different backends to use for
storing modifications to the zero bootstrap config. The two currently
implemented backends allow writing changes to a file or to a Kubernetes
secret. Backend selection is determined by the scheme in a URI passed to
the flag '--config-writeback-uri'.

In a Kubernetes environment, where the bootstrap config is mounted into
the pod from a secret, this option allows Pomerium to write changes back
to the secret, as writes to the mounted secret file on disk are not
persisted.

* Use env vars for bootstrap config filepath/writeback uri

* linter pass and code cleanup

* Add new config writer options mechanism

This moves the encryption cipher parameter out of the WriteConfig()
method in the ConfigWriter interface and into a new ConfigWriterOptions
struct. Options (e.g. cipher) can be applied to an existing ConfigWriter
to allow customizing implementation-specific behavior.

* Code cleanup/lint fixes

* Move vendored k8s code into separate package, and add license header and package comment

internal/zero/bootstrap/source_test.go

@@ -18,7 +18,7 @@ func TestConfigChanges(t *testing.T) {
 
 	secret := []byte("secret")
 
-	src, err := bootstrap.New(secret, nil, nil)
+	src, err := bootstrap.New(secret, nil, nil, nil)
 	require.NoError(t, err)
 
 	ptr := func(s string) *string { return &s }