Suggestion: include Doltgres brand in version string

Currently, the return value of version() is something like `PostgreSQL 15.5`. It would be useful to include `Doltgres` in the string returned by function version(), and possibly in a parameter string sent on PostgreSQL connection startup such as `database=doltgres`.

Rationale: clients connecting using the PostgreSQL wire protocol may need to change their behaviour depending on the PostgreSQL variant. For example, Doltgres doesn't currently parse timezones such as `UTC-01:00`.

Examples for other PostgreSQL variants:

- CockroachDB has CockroachDB CCL v26.1.1 (x86_64-pc-linux-gnu, built 2026/03/04 12:08:22, go1.25.5)
- YDB has backend parameter server_version=14.5 (ydb stable-23-4)
- Yugabyte has PostgreSQL 15.12-YB-2025.2.1.0-b0 on x86_64-pc-linux-gnu, compiled by clang version 19.1.0 (https://github.com/yugabyte/llvm-project.git a2a6b655e14e7fa1fcf1011a6cb29cb8575249c0), 64-bit
- Materialize has PostgreSQL 9.5 on x86_64-unknown-linux-gnu (Materialize 26.16.0), startup parameter database=materialize
- Risingwave has PostgreSQL 13.14.0-RisingWave-2.8.0 (bb13a21d5d5982e947887ce794b4bff99f109fc7)