Add created attribute to image_manifest rule

[malt3]

This adds a more ergonomic way to set the image creation timestamp compared to using config_fragment. The created attribute accepts a file containing an RFC 3339 formatted datetime string, making it easier to integrate with Bazel's stamping mechanism.

Changes:

• Add created attribute to image_manifest rule in manifest.bzl
• Update Go manifest command to accept --created flag
• Add readCreatedTimestamp function to parse RFC 3339 timestamps
• Set config.Created field when timestamp is provided
• Add test case in e2e/generic demonstrating usage

Closes #203

[aspect-workflows]

Bazel 8 (Test)

4 test targets passed

Targets

//docs:update_0_test [k8-fastbuild]                                                    112ms
//img/private/release/gazelle_plugin:release_files_test [k8-fastbuild]                 270ms
//tests/img_toolchain:img_toolchain_test [k8-fastbuild]                                689ms
//util:buildifier.check [k8-fastbuild]                                                 1s

Total test execution time was 2s. 11 tests (73.3%) were fully cached saving 793ms.

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Integration Tests (Test)

17 test targets passed

Targets

//e2e:cc_bazel_7_4_0                     4m 55s
//e2e:cc_bazel_8_4_2                     4m 55s
//e2e:cc_bazel_9_0_0-pre_20251022_1      5m 2s
//e2e:generic_bazel_7_4_0                1m 20s
//e2e:generic_bazel_8_4_2                1m 14s
//e2e:generic_bazel_9_0_0-pre_20251022_1 1m 28s
//e2e:go_bazel_7_4_0                     3m 7s
//e2e:go_bazel_8_4_2                     3m 4s
//e2e:go_bazel_9_0_0-pre_20251022_1      3m 11s
//e2e:js_bazel_7_4_0                     40s
//e2e:js_bazel_8_4_2                     36s
//e2e:js_bazel_9_0_0-pre_20251022_1      40s
//e2e:python_bazel_7_4_0                 1m 47s
//e2e:python_bazel_8_4_2                 1m 42s
//e2e:python_bazel_9_0_0-pre_20251022_1  1m 43s
//e2e:workspace_bazel_7_4_0              15s
//e2e:workspace_bazel_8_4_2              19s

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Buildifier