microsoft: support client_credentials flow using client assertions

[manicminer]

Resolves #465

Usage

conf := microsoft.Config{
	ClientID:    clientId,
	PrivateKey:  []byte("my_encoded_private_key"),
	Certificate: []byte("my_encoded_certificate"),
	Scopes:      []string{"https://graph.microsoft.com/.default"},
	TokenURL:    microsoft.AzureADEndpoint(tenantId).TokenURL,
}

ts := conf.TokenSource(context.Background())

Documentation

• https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-oauth2-client-creds-grant-flow#second-case-access-token-request-with-a-certificate
• https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-certificate-credentials

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This PR (HEAD: e81c58e) has been imported to Gerrit for code review.

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