feat: max_pool_size alias for default_pool_size (and pool_size)

[levkk]

Create a default_pool_size alias in config:

# both settings are identical
[general]
max_pool_size = 100
default_pool_size = 100

[[databases]]
name = "prod"
host = "10.0.0.0"
# both settings are identical
pool_size = 100
max_pool_size = 100

[codecov]

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