hcl2cdk: convert top-level ephemeral {} blocks

[so0k]

Follow-up from #296 (tracked in the PR body as "hcl2cdk conversion of ephemeral {} blocks — tracked, not built").

Why

#296 generates TerraformEphemeralResource bindings and synthesizes top-level ephemeral {} blocks, but the convert direction is missing: cdktn convert silently drops ephemeral blocks from input HCL. packages/@cdktn/hcl2cdk/src/schema.ts defines the accepted input via a non-passthrough zod object; unknown top-level keys (including ephemeral) are stripped before conversion logic runs, with no warning.

What

cdktn convert should turn ephemeral "random_password" "pw" { ... } into new EphemeralRandomPassword(...) referencing the generated bindings, exactly parallel to resource/data-source conversion.

How (implementation notes)

• schema.ts: add ephemeral top-level key (same shape as resource).
• Conversion path: mirror the data-source mapping with the ephemeral_ type prefix used by the generator (class names Ephemeral<PascalType>, namespace folders ephemeral-<kebab-type>/) — the prefix conventions were fixed in feat: support newer provider plugin-protocol features via targetVersions (RFC-04) #296's provider-generator commit.
• References: ephemeral.<type>.<name>.<attr> expressions must resolve to the construct's attribute accessors.
• Consider emitting a conversion warning for any other silently-dropped top-level key while in there (moved/check/removed share the same silent-strip behavior).