Add OCI KMS as provider

[hrvolapeter]

Add functionality for Oracle Cloud (OCI) and it's KMS.

Implements authentication using DEFAULT profile which is read from ~/.oci/config file

[felixfontein]

Related: #1226

[b-dean]

I'd rather not have this just be the default config provider. Here's some reasons:

1. It doesn't work with any sort of standard environment variables. The TF_VAR_* stuff is in there, but no OCI_* or OCI_CLI_*
2. It doesn't work with the security_token auth that you get when you use SSO / SAML / Federation stuff.
3. It requires you always create a ~/.oci/config which is a pain when you're running somewhere temporary, like in a container, and might not have proper users, etc.

I've written a small library that could help: https://github.com/ontariosystems/oci-cli-env-provider

It uses the semi-standard OCI_CLI_* environment variables used by the oci cli.

So what I'd do here is change it get the provider that way, and include the instance identity provider so we can run it on compute instances too (which I definitely need sops to be able to do).

import "github.com/ontariosystems/oci-cli-env-provider"

func (key *MasterKey) newKMSClient() (*kms.KmsCryptoClient, error) {
	configProvider, err := configurationProvider()
	if err != nil {
		return nil, err 
	}

	// ...
}

func configurationProvider() (common.ConfigurationProvider, error) {
	var providers []common.ConfigurationProvider

	providers = append(providers, ocep.DefaultConfigProvider())
	ip, err := auth.InstancePrincipalConfigurationProvider()
	if err != nil {
		return nil, err
	}
	providers = append(providers, ip)

	return common.ComposingConfigurationProvider(providers)
}

I've added a similar comment on #1226