[Hammer] Support long-lived performance tests with private GCS bucket

[roger2hk]

Description

The existing hammer takes bearer token for read and write. However, the token can expire in 1 hour by default. The access token can be extended to 12 hours.

https://github.com/transparency-dev/trillian-tessera/blob/f55995cb1acaff9c64c95fec24ebccdecccd9c49/internal/hammer/hammer.go#L64-L65

When the token is expired, the read access is revoked. Thus, all requests to the bucket including checkpoint and tiles will return 401.

W0301 01:13:15.008988    2604 hammer.go:102] get("https://storage.googleapis.com/bucket/checkpoint"): 401

Supporting gs://bucket_name/file_name scheme could be one of the solutions. Another workaround could be mounting the bucket to the local file system and use the file:// scheme.