Fix concurrency issues

[pierluc-codes]

🛠️ Description

Fixes parallelism problem where multiple token exchange happen if the code is invoked by multiple go routine. The lock also act as a protector for the cached file on the filesystem.

The crash can be reproduced by simulating a slower than expected response from the oauth webserver with the the following patch

diff --git a/auth/browser.go b/auth/browser.go
index fc10933..8559cfe 100644
--- a/auth/browser.go
+++ b/auth/browser.go
@@ -87,6 +87,7 @@ func (b *oauthBrowser) GetTokenFromBrowser(ctx context.Context, conf *oauth2.Con
 			return nil, err
 		}
 
+		time.Sleep(time.Second * 4)
 		err = callbackEndpoint.server.Shutdown(context.Background())
 		if err != nil {
 			return nil, fmt.Errorf("failed to shutdown callback server: %w", err)

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👍 Definition of Done

• SDK added
• SDK updated
• Tests added?
• Docs updated?