sdk: don't import Kubernetes packages

[msanft]

The SDK previously caused quite large code to be generated. The following test program results in a 50MB binary, for example:

package main

import (
	"context"
	"fmt"
	"net/http"
	"os"

	"github.com/edgelesssys/contrast/sdk"
	"github.com/spf13/afero"
)

func main() {
	nonce := make([]byte, 32)
	client := sdk.New().
		WithFSStore(afero.NewBasePathFs(afero.NewOsFs(), os.TempDir())).
		WithHTTPClient(http.DefaultClient)

	att, err := client.GetAttestation(context.Background(), "https://example.com", nonce)
	if err != nil {
		panic(err)
	}

	valid, err := client.ValidateAttestation(context.Background(), nonce, att)
	if err != nil {
		panic(err)
	}
	fmt.Println(valid)
}

When analyzing the code with go-size-analyzer, it becomes apparent that most of the size stems from Kubernetes Go packages:

Note that this doesn't even include contributions of those packages to .rodata and other sections for constants, strings, etc.

@thomasten managed to trace this back to the history package a while ago. This PR now addresses the bloat by splitting the subset of history that requires the Kubernetes imports, namely the ConfigMap-backed store, into a separate package. As Go links on package level, this is necessary to have it not be linked into the produced binaries. A separate commit then aligns the rest of the history package with those naming changes, extracting the other, Afero-backed store into a separate package. This change reduces the size of the test binary vom 50MB to 18MB, a reduction of almost 70%!

See the commit messages for the individual technical details.

[burgerdev]

Thanks for fixing this!

[burgerdev]

lgtm

[burgerdev]

There's still an inverse dependency here, but I can try to consolidate that later.

[msanft]

This is from RecoverConfigMaps, which I'm not totally sure where to move to. Thanks!