Add ImageSpec.from_env

[thomasjpfan]

Tracking issue

Closes flyteorg/flyte#5889

Why are the changes needed?

Resolve notebook environment mismatch described in flyteorg/flyte#5889

What changes were proposed in this pull request?

This PR adds a ImageSpec.from_env into ImageSpec.

How was this patch tested?

I added unit test to this PR and ran this workflow:

from pathlib import Path

from flytekit import current_context, task, workflow, ImageSpec
from flytekit.types.file import FlyteFile

image_spec = ImageSpec.from_env(
    pinned_packages=["joblib"],
    name="myimage",
    registry="localhost:30000",
    packages=[
        "git+https://github.com/thomasjpfan/flytekit@41a6a74df4bab1b7db4f9ff13eb249e94fe7a2e4"
    ],
    apt_packages=["git"],
)


@task(container_image=image_spec)
def create_file() -> FlyteFile:
    working_dir = Path(current_context().working_directory)

    file = working_dir / "outer_file.txt"
    file.write_text("this is some text")

    return file


@workflow
def main() -> FlyteFile:
    return create_file()

[codecov]

Codecov Report

Attention: Patch coverage is 15.78947% with 16 lines in your changes missing coverage. Please review.

Project coverage is 46.62%. Comparing base (6f8b9a0) to head (3712300).

Files with missing lines	Patch %	Lines
flytekit/image_spec/image_spec.py	15.78%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2895      +/-   ##
==========================================
- Coverage   46.81%   46.62%   -0.19%     
==========================================
  Files         200      200              
  Lines       20901    20854      -47     
  Branches     2689     2692       +3     
==========================================
- Hits         9785     9724      -61     
- Misses      10635    10653      +18     
+ Partials      481      477       -4     

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