Implement lambda function import

[t0yv0]

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Issue details

Currently lambda functions created with Pulumi assets or archives fail to import cleanly into Pulumi, specifically Pulumi does not recover the original source code or source package from the cloud and repackage it into the Pulumi program. Users that need to import lambda functions currently end up resorting to workarounds and it is not a good experience see for example the discussion below and in the associated PR:

#2392

• only sourceCodeHash is imported
• this runs amok newly upstream introduced ExactlyOnce validators since sourceCodeHash is expected alongside filename
• the workaround suggested for Terraform of supplying a dummy filename combined with "ignoreChanges" does not work in Pulumi since ignoreChanges copies the empty state value before it hits TF validators
• we currently workaround by patching upstream to weaken the validators

There's some indication that upstream is not fixing it in and has related issues:

• Trouble Importing Lambda Function with Terraform's New Import Block hashicorp/terraform#34374
• source_code_hash does not update hashicorp/terraform-provider-aws#7385

Option 1: document how to actually import lambdas

We could at least work through what's the recommended way to import lambda functions in this provider is, and document it, possibly enhancing error messages to guide the user. It seems that recovering the source package should be possible from the console. Trying to see if there is a programmatic way as well? https://docs.aws.amazon.com/cli/latest/reference/lambda/#cli-aws-lambda does not seem to indicate it. If the code can be recovered, it can be turned into an S3 object and the function can then be specified in Pulumi using the indirect via Amazon S3 (using the s3_bucket, s3_key and s3_object_version arguments) method.

Option 2: build smooth automatic import

If recovering the source can be done programmatically, we can automate this all so that the import results in a new s3 object and a lambda code that references it using s3_key.

Affected area/feature