chore: loosen dependency requirements for base64

[wikiwong]

Attempts to address an error seen when upgrading both Rust PDK & SDK to latest:

    Updating git repository `https://github.com/extism/extism.git`
    Updating git repository `https://github.com/extism/rust-pdk.git`
    Updating crates.io index
error: failed to select a version for `base64`.
    ... required by package `extism-convert v0.2.0 (https://github.com/extism/extism.git?branch=main#7636c873)`
    ... which satisfies git dependency `extism-convert` of package `extism v1.0.0-alpha.0 (https://github.com/extism/extism.git?branch=main#7636c873)`
    ... which satisfies git dependency `extism` of package `proto_core v0.22.4 (/Users/miles/Projects/proto/crates/core)`
    ... which satisfies path dependency `proto_core` (locked to 0.22.4) of package `proto_cli v0.22.0 (/Users/miles/Projects/proto/crates/cli)`
versions that meet the requirements `^0.21.3` are: 0.21.5, 0.21.4, 0.21.3

all possible versions conflict with previously selected packages.

  previously selected package `base64 v0.21.0`
    ... which satisfies dependency `base64 = "^0.21.0"` (locked to 0.21.0) of package `extism-manifest v0.5.0`
    ... which satisfies dependency `extism-manifest = "^0.5.0"` (locked to 0.5.0) of package `extism-pdk v1.0.0-beta.0 (https://github.com/extism/rust-pdk.git?branch=main#009bf808)`
    ... which satisfies git dependency `extism-pdk` of package `proto_pdk v0.10.2 (/Users/miles/Projects/proto/crates/pdk)`

failed to select a version for `base64` which could resolve this conflict