Add Lambda environment variables for the Asset Proxy feature

[pjhartzell]

An asset proxy feature was added to stac-server in stac-utils/stac-server#991. Three environment variables need to be added to the API and Ingest Lambda functions for full feature functionality:

• ASSET_PROXY_BUCKET_OPTION: One of NONE, ALL, ALL_BUCKETS_IN_ACCOUNT, or LIST (default: NONE, which disables the Asset Proxy feature)
• ASSET_PROXY_BUCKET_LIST: Required only when ASSET_PROXY_BUCKET_OPTION is LIST. Value should be a comma delimited string, e.g., 'bucket1,bucket2,bucket3'.
• ASSET_PROXY_URL_EXPIRY: Pre-signed URL expiry in seconds (default: 300)