Review default features

[gsouf]

Hi,

In cargo.toml, both of actix and axum are enabled as default features:

default = ["actix-web", "axum", "openssl"]

At the same time, in the readme, the recommended installation approach for actix is:

[dependencies]
firebase-auth = { version = "<version>", features = ["actix-web"] }
actix-web = "4"

and axum:

[dependencies]
firebase-auth = { version = "<version>", features = ["axum"] }
axum = "0.8"

With this setup if for example I use axum, it will also pull actix in the project, and vice versa.

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I think the optimal solution is that none of axum and actix should be enabled as default features.

Thanks for your work on this project!