chore: refactor codersdk to use SessionTokenProvider (#19565)

Refactors `codersdk.Client`'s use of session tokens to use a `SessionTokenProvider`, which abstracts the obtaining and storing of the session token.

The main motiviation is to unify Agent authentication an an upstack PR, which can use cloud instance identity via token exchange, rather than a fixed session token.

However, the abstraction could also allow functionality like obtaining the session token from other external sources like the OS credential manager, or an external secret/key management system like Vault.

coderd/coderdtest/oidctest/idp.go

@@ -641,7 +641,7 @@ func (f *FakeIDP) LoginWithClient(t testing.TB, client *codersdk.Client, idToken
 
 // ExternalLogin does the oauth2 flow for external auth providers. This requires
 // an authenticated coder client.
-func (f *FakeIDP) ExternalLogin(t testing.TB, client *codersdk.Client, opts ...func(r *http.Request)) {
+func (f *FakeIDP) ExternalLogin(t testing.TB, client *codersdk.Client, opts ...codersdk.RequestOption) {
 	coderOauthURL, err := client.URL.Parse(fmt.Sprintf("/external-auth/%s/callback", f.externalProviderID))
 	require.NoError(t, err)
 	f.SetRedirect(t, coderOauthURL.String())
@@ -660,11 +660,7 @@ func (f *FakeIDP) ExternalLogin(t testing.TB, client *codersdk.Client, opts ...f
 	req, err := http.NewRequestWithContext(ctx, "GET", coderOauthURL.String(), nil)
 	require.NoError(t, err)
 	// External auth flow requires the user be authenticated.
-	headerName := client.SessionTokenHeader
-	if headerName == "" {
-		headerName = codersdk.SessionTokenHeader
-	}
-	req.Header.Set(headerName, client.SessionToken())
+	opts = append([]codersdk.RequestOption{client.SessionTokenProvider.AsRequestOption()}, opts...)
 	if cli.Jar == nil {
 		cli.Jar, err = cookiejar.New(nil)
 		require.NoError(t, err, "failed to create cookie jar")