extra_float_digits URL support

[brianheineman]

When attempting to use SQLx 0.7.1 with Redshift, an error is returned upon connection due to extra_float_digits defaulting to 3. Adding the capability to define this value via a URL parameter removes the need for code changes or work-arounds (e.g. #801).

Connection Error: error returned from database: 3 is outside the valid range for parameter "extra_float_digits" (-15 .. 2)

[abonander]

Honestly, I'd rather just lower the default to 2. From Postgres 12 onward, any positive value selects the shortest-precise format so it doesn't really matter what we pass: https://www.postgresql.org/docs/12/runtime-config-client.html#GUC-EXTRA-FLOAT-DIGITS

The last Postgres release to actually care about the exact value will sunset in less than two months, and this behavior only ever affected the text-format output of floats anyway.

Otherwise my opinion is the same as #2135: it doesn't belong in the URL.

If you'd like to open a PR for changing the default, feel free.

[brianheineman]

Thanks @abonander, I believe that is a much better solution as it will not require configuration when used with Redshift. I opened #2832 per your recommendation.