Enable getting envvars from `.env` file

[mfisher87]

Currently we use os.environ to access the envvars directly, which doesn't care about a .env file. If we use either dotenv library or pydantic-settings (my personal preference!) we can automatically load envvars from a .env file. pydantic-settings offers additional encapsulation of valdation and derived values.

[Sherwin-14]

@mfisher87 I can take this up for now, do you have any specific opinions on this?

[mfisher87]

Awesome, thanks Sherwin! Only preferences I can think of right now are what's stated in the issue description. ❤️

[chuckwondo]

@mfisher87, based on the conversation in #1265, it appears that at least a couple of folks are rejecting that PR (@jhkennedy and @chuckwondo), and by extension, effectively flagging this issue as "close as not planned," but I wanted to bring this to your attention before taking action in that direction.