[Feature]: unify config across backends

[evacchi]

Problem Statement

currently each backend uses different sets of flags that are partially overlapping.

• Some backends allow config files, some backends have shortcuts to set flags on the CLI (e.g. SortedSet), etc.
• gates are parsed differently across backends (e.g. sorted set uses a custom StringMap; GCP PubSub uses map[string]string; composite gate defines its own local struct)
• config surfaces are fragmented: queue/topic configs, pool configs, runtime params, and gate configs are spread across separate files, CLI flags, and env vars with different loading patterns
• only one transport can be active per async instance, but the config flags do not make this explicit

Proposed Solution

• We introduce a single config flag to set the backend (e.g. --transport) and we pass either a --transport-config [string] or a --transport-config-file [path] + a --pool-config-file
• we allow some env variables to override specific keys (e.g. redis url etc) to specify secrets
• we extract shared parameter passing and pass structured data to the backend constructors

Alternatives Considered

n/a

Willingness to Contribute

Yes, I can submit a PR

Additional Context

this is a somewhat significant refactor and might be introduce some breakage, so we should be mindful about that and possibly introduce some temporary measure for backwards compatibility if necessary