add support for Kubernetes TLS keys for flux create secret tls

Add support for using `tls.key`, `tls.crt` and `ca.crt` keys while
generating a Secret, using the `--tls-key-file`, `--tls-crt-file` and
`--ca-crt-file` flags respectively.
Mark the flags `--key-file`, `--cert-file` and `--ca-file` as
deprecated.

Signed-off-by: Sanskar Jaiswal <jaiswalsanskar078@gmail.com>

cmd/flux/testdata/create_secret/tls/secret-tls.yaml

@@ -5,7 +5,19 @@ metadata:
   name: certs
   namespace: my-namespace
 stringData:
-  certFile: |
+  ca.crt: |
+    -----BEGIN CERTIFICATE-----
+    MIIBhzCCAS2gAwIBAgIUdsAtiX3gN0uk7ddxASWYE/tdv0wwCgYIKoZIzj0EAwIw
+    GTEXMBUGA1UEAxMOZXhhbXBsZS5jb20gQ0EwHhcNMjAwNDE3MDgxODAwWhcNMjUw
+    NDE2MDgxODAwWjAZMRcwFQYDVQQDEw5leGFtcGxlLmNvbSBDQTBZMBMGByqGSM49
+    AgEGCCqGSM49AwEHA0IABK7h/5D8bV93MmEdhu02JsS6ugB8s6PzRl3PV4xs3Sbr
+    RNkkM59+x3b0iWx/i76qPYpNLoiVUVXQmA9Y+4DbMxijUzBRMA4GA1UdDwEB/wQE
+    AwIBBjAPBgNVHRMBAf8EBTADAQH/MB0GA1UdDgQWBBQGyUiU1QEZiMAqjsnIYTwZ
+    4yp5wzAPBgNVHREECDAGhwR/AAABMAoGCCqGSM49BAMCA0gAMEUCIQDzdtvKdE8O
+    1+WRTZ9MuSiFYcrEz7Zne7VXouDEKqKEigIgM4WlbDeuNCKbqhqj+xZV0pa3rweb
+    OD8EjjCMY69RMO0=
+    -----END CERTIFICATE-----
+  tls.crt: |
     -----BEGIN CERTIFICATE-----
     MIIFazCCA1OgAwIBAgIUT84jeO/ncOrqI+FY05Fzbg8Ed7MwDQYJKoZIhvcNAQEL
     BQAwRTELMAkGA1UEBhMCQVUxEzARBgNVBAgMClNvbWUtU3RhdGUxITAfBgNVBAoM
@@ -37,7 +49,7 @@ stringData:
     KvJGPigx4fAwfR/d78o5ngwoH9m+8FUg8+qllJ8XgIbl/VXKTk3G4ceOm4eBmrel
     DwWuBhELSjtXWPWhMlkiebgejDbAear53Lia2Cc43zx/KuhMHBTlKY/vY4F2YiI=
     -----END CERTIFICATE-----
-  keyFile: |
+  tls.key: |
     -----BEGIN PRIVATE KEY-----
     MIIJRAIBADANBgkqhkiG9w0BAQEFAASCCS4wggkqAgEAAoICAQDn/rPsZ74oypiw
     CzLlx57zplTiCi/WLSF+MmLGuTvMEQnV+OND2zFgvDIV/vFs3brkd6rLVI4NcdgS